The present invention relates to the lighting arts. It is particularly applicable to the fabrication of high-brightness gallium nitride (GaN) based light emitting diodes (LEDs) and LED arrays, and will be described with particular reference thereto. However, the invention also finds application in connection with other types of LEDs and in other LED applications.
With reference to FIG. 1, a conventional gallium nitride (GaN) based LED 10 includes thin layers of semiconductor material of two opposite conductivity types, typically referred to as p-type layers 12 and n-type layers 14. The layers 12, 14 are typically arranged in a stack, with one or more layers of n-type material in one part of the stack and one or more layers of p-type material at an opposite end of the stack. The LED 10 includes a light-emitting p-n junction region 16 arranged between the p-type layers 12 and the n-type layers 14. The various layers of the stack are deposited on a substrate 18, such as a sapphire substrate, by metal-organic vapor deposition (MOCVD), molecular beam epitaxy (MBE), or another deposition technique. After deposition, the substrate is typically cut or diced to form a plurality of LED packages. Each package includes one or more LEDs and a portion of the substrate 18.
In operation, an electric current passed through the LED 10 using electrical contacts 19 is carried principally by electrons in the n-type layer 14 and by electron vacancies or “holes” in the p-type layer 12. The electrons and holes move in opposite directions toward the junction layer 16, where they recombine with one another. Energy released by the electron-hole recombination is emitted from the LED 10 as light 20. As used herein, the term “light” includes visible light as well as electromagnetic radiation in the infrared and ultraviolet wavelength ranges. The wavelength of the emitted light 20 depends upon many factors, including the composition of the semiconductor materials, the structure of the junction 16, the presence or absence of impurities in the junction 16, and the like.
GaN-based LEDs, such as the LED 10 shown in FIG. 1, are typically fabricated on sapphire substrates 18, through which substrate 18 light can be extracted from a substrate back side 21. Alternatively, it is known to use a reflective layer 22 applied to the back side 21 of the LED 10. The reflective layer 22 reflects the emitted light 20 to produce reflected light 23 that contributes to a front-side light output and improves light extraction from the LED. Typically, the reflective contact is comprised of a single layer of aluminum or gold deposited on the back side 21 of the substrate 18. Such a configuration is illustrated in U.S. Pat. No. 5,939,735 issued to Tsutsui et al.
GaN-based LEDs are conventionally attached to a lead frame or a heat sink using a die-attach epoxy between the back surface 21 or the reflective layer 22 and the lead frame or heat sink. A single aluminum layer reflective contact on a GaN-based LED as proposed by Tsutsui only allows die attachment using an adhesive epoxy compound.
The use of epoxy to attach an LED die to a lead frame or heat sink causes problems. First, die-attach epoxies typically have a low thermal conductivity resulting in a thermal resistance between the active region of the LED and the heat sink of approximately 120° C./W. Such a high thermal resistance limits the amount of current and/or power which can safely be applied to the LED without encountering failure due to overheating or the like.
Second, epoxy compounds are subject to degradation when illuminated by a blue or ultraviolet light produced by the LED. This degradation is more pronounced at the elevated temperatures typically encountered in high-brightness GaN LED operation.
In view of the disadvantages of epoxies for connecting LED chips to a heat sink, lead frame or the like, it would be preferable to employ a solder connection. Solder connections typically exhibit a low thermal resistance of about 20° C./W and possibly as low as 5° C./W. Furthermore, there are several package configurations which are particularly well-suited for soldering of the LED. Generally, reflective layers and/or contacts such as those proposed by Tsutsui and/or shown in FIG. 1 are incompatible with soldering because the aluminum does not provide a good surface for solder bonding. Similarly, a gold reflective layer adheres weakly to the sapphire substrate, and so soldering to a gold reflective layer typically results in delaminating of the gold layer from the substrate.
The present invention contemplates an improved backside metallization and method for forming the same that overcomes the above-mentioned limitations and others.